found some old files in /var/portage/distfiles and /var/tmp/portage/x11-wm that I removed. Prehaps from a very old install attempt, altho when I tried emerge --unmerge fvwm first it returned nothing, and portage was listing fvwm as not installed.

After that I did an emerge -eav portage. Probably didn't have to, but just to be on the safe side. Now fvwm is running happily. Marking as solved._________________SPARC Ultra 80, 4x 440mhz, 4 gigs ram, 2x Creator 3D video card, 36 gig SCSI, SCSI CDROM, running Gentoo 2.6.21.